This disclosure concerns a method of bonding a thin film of diamond to a second thick diamond substrate in a way that does not cause the exposed (un-bonded) diamond surface to become contaminated by the bonding process or when the bonded diamond is held at high temperature for many hours in vacuum.
In addition, this process allows the thin film diamond to be bonded over a hole in the substrate diamond.
In addition, the process provides a metal surface suitable for wire bonding on the surface of the thick diamond substrate adjacent to the thin film diamond but electrically isolated from it.
In addition, the process provides a bond material that is inert enough to withstand subsequent processing such as photo-resist development and most chemical etches.
Common bonding methods include thermo-compression bonding and brazing. Thermo-compression bonding requires application of very high pressure at high temperature, and is thus difficult to accomplish for a fragile part. More fundamentally, metals such as Au, that are most commonly used for thermo-compression bonding, exhibit fast surface diffusion, which could create an electric short circuit and/or contaminate the NEA surface.
Brazing requires that a filler metal be melted to create the bond, such that the melting point of the metal must be well above the intended operating temperature. However, the vapor pressures of suitable metals at their melting points are high enough to contaminate the diamond. When contaminated with metals at high temperature, the diamond typically reacts with the metal, forming metal carbides that are relatively inert and difficult to remove chemically without also removing the bond metal. For example, braze tests using Cu:Ag (72:28 wt %) eutectic alloy, which melts at 779° C., resulted in large amounts of both Cu and Ag on the exposed diamond surface, and these metals could not be completely removed using solutions that dissolve pure Cu and pure Ag. Other metals and alloys also have high vapor pressure at their melting points. Thus brazing is not an appropriate bonding method.